Lately I’ve been exchanging riddles / mind puzzles with some coworkers of mine. I’m not really sure how it started, but one ended up on my white board at work and before I knew it everyone wanted to know the answer or had their own riddle to challenge me with. I’ve collected up some of my favorites. If you’re unfamiliar with one and you solve it, feel free to post your proposed solution as a comment.

As far as site updates go, I have some coming, but I’ve had a distracting past couple of weeks. Since January I’ve been suffering from tinnitus (ringing in the ears), but it didn’t really get bothersome until the middle of last month. I ended up going on a huge research quest for a cure, but it appears there are only things you can take to prevent tinnitus (NAC, antioxidants) and nothing that’ll really cure it. Right now it’s at a volume I can deal with, so I’m fine, but I want to caution everyone and let you know its important to take care of your hearing. Also, avoid things that are too good to be true. There seems to be a lot of snake oil out there when it comes to tinnitus.

Anyway, enough of my personal problems, on with the riddles…

1) In front of you there are ten boxes of books and a digital scale large enough and strong enough to hold all of the boxes when they’re filled with books. Inside of each box is ten books. In nine of the boxes, the books weigh 1.0 pounds each. In one of the boxes the books weigh 0.9 pounds each. Using the digital scale only once, how do you determine the box that contains the books that weigh 0.9 pounds? You may take the books out of the boxes and you can assume the weight of each box is 1 pound.

2) You’re one an island and you want to get off of it. There are two caves off the island. One leads to certain doom while the other leads off the island. In front of the caves there are two guards. One always tells the truth and the other always lies. You do not know which one is which. You may ask one question which they both will answer. What question can you ask that will guarantee you pick the right cave to get off of the island? [Editors Note: After much discussion, we discovered there were actually multiple answers to this question, so if you know the answer, try to come up with another one]

3) Four men are standing in a row. They are ordered tallest to shortest. The tallest man is facing west while the other three men are facing east. Each man is wearing a hat that is either black or white. Each man knows that there are 2 black hats and 2 white hats, however, they do not know the color of the hat that they are wearing. The tallest man cannot see anyone’s hat, the next tallest man can see the hat color of the two men in front of him, the next tallest man can see the hat color of the man in front of him, and the shortest man can see no one’s hat color. Imagine that the hat ordering from tallest to shortest is: black, white, black, white. Where the tallest person is wearing a black hat and the shortest person is wearing a white hat.

None of these men can move around, however, they can talk to each other – though they do not have to be truthful when answering each other’s questions. As soon as a man figures out his hat color he screams it out (and he cannot scream unless he really knows his hat color). Which man can figure out his hat color and why? [Editors Note: This one is easier if you draw it]

4) You have 25 runners. When raced, they always perform the same, and no two runners perform identically. You can only race 5 at a time, and once a race is over, the only information you get is the order that they finished in. How do you determine the fastest, second fastest, and third fastest runner using only seven races?

5) This was the puzzle that started it all. No one I’ve told it to has been able to figure it out. And it always takes a while for the answer to make sense.